Write a program which accepts a valid infix expression and provide its corresponding postfix expression. Consider following assumptions:
• There are 5 operators (+,-,*,/ and ^).
• + and – follow right to left associativity
• * and / also follow right to left associativity
• ^ follow left to right associativity
#include<stdio.h>
#include<ctype.h>
char stack[20];
int top = -1;
void push(char x)
{
stack[++top] = x;
}
char pop()
{
if(top == -1)
return -1;
else
return stack[top--];
}
int priority(char x)
{
if(x=='^')
return 0;
if(x == '+' || x == '-')
return 1;
else if(x == '*' || x == '/')
return 2;
}
void main()
{
char exp[20];
char *e, x;
printf("Enter the expression :: ");
scanf("%s",exp);
e = exp;
while(*e != '\0')
{
if(isalnum(*e))
printf("%c",*e);
else if(*e=='^')
{
while(priority(stack[top]) >=priority(*e))
printf("%c",pop());
push(*e);
}
else
{
while(priority(stack[top]) > priority(*e))
printf("%c",pop());
push(*e);
}
e++;
}
while(top != -1)
{
printf("%c",pop());
}
}
This program gives an infinite loop upon execution.
I get following output after entering any expression,
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▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
When there is no element in the stack i.e. top = -1 in your code and you call priority on that it return garbage + IMO top should be returning the no. of elements in the stack for ease rather than position in array.
Apart from this the changed associativity will not serve the purpose as a-b is not equal to b-a usually.
Here is the corrected code:
#include<stdio.h>
#include<ctype.h>
char stack[20];
int top = 0; //CHANGE 1 -NOW IT MEANS NO. OF ELEMENTS IN THAT STACK
void push(char x)
{
stack[top++] = x;
}
void swap(char c[],int i, int j)
{
char temp;
temp =c[i];
c[i] = c[j];
c[j] = temp;
}
char pop()
{
return stack[--top]; //CHANGE 2
}
int priority(char x)
{
if(x=='^')
return 3; //CHANGE 3 - CONCEPTUAL
if(x == '+' || x == '-')
return 1;
else if(x == '*' || x == '/')
return 2;
return -1;
}
int main()
{
char exp[20];
char *e, x;
printf("Enter the expression :: ");
scanf("%s",exp);
//CHANGE INDUCED AS ASSOCIATIVITY HAS BEEN REVERSED FOR OPERATORS OTHER THAN '^'
int i=0,length=strlen(exp);
while(i<length/2) //REVERSING THE INPUT EXPRESSION
{
swap(exp,i,length-i-1);
++i;
}
e = exp;
while(*e != '\0')
{
if(*(e+1)=='^') //ASSOCIATIVITY OF THIS OPERATOR HAS BEEN KEPT AS USUAL HENCE REVERSING A SMALL PART JUST TO COMPUTE IT AS USUAL
{
swap(e,0,2);
}
x=*e; //CHANGE 4
if(isalnum(x))
printf("%c",x);
else
{
while((top>0)&&(priority(stack[top-1]) > priority(x))) //CHANGE 5
printf("%c",pop());
push(x);
}
e++;
}
while(top>0) //CHANGE 5
{
printf("%c",pop());
}
return 0;
}
INPUT:
a+b^c
cab^-
a+b*c^d
OUTPUT (with reversed associativity of operators except '^'):
1.
bc^a+
ab^c+
cd^b*a+
Related
#include<stdio.h>
#include<stdlib.h>
#include<ctype.h>
#include<string.h>
#define SIZE 100
char stack[SIZE];
int top = -1;
void push(char item)
{
if(top >= SIZE-1)
{
printf("\nStack Overflow.");
}
else
{
top = top+1;
stack[top] = item;
}
}
char pop()
{
char item;
if(top <0)
{
printf("stack under flow: invalid infix expression");
}
else
{
item = stack[top];
top = top-1;
return(item);
}
}
int is_operator(char symbol)
{
if(symbol == '^' || symbol == '*' || symbol == '/' || symbol == '+' || symbol =='-')
{
return 1;
}
else
{
return 0;
}
}
int precedence(char symbol)
{
if(symbol == '^')
{
return(5);
}
else if(symbol == '/')
{
return(4);
}
else if(symbol == '*')
{
return(3);
}
else if(symbol == '+' )
{
return(2);
}
else if(symbol == '-' )
{
return(1);
}
else
{
return(0);
}
}
void InfixToPostfix(char infix_exp[], char postfix_exp[])
{
int i, j;
char item;
char x;
push('(');
strcat(infix_exp,")");
i=0;
j=0;
item=infix_exp[i];
while(item != '\0')
{
if(item == '(')
{
push(item);
}
else if( isdigit(item) || isalpha(item))
{
postfix_exp[j] = item; /* add operand symbol to postfix expr */
j++;
}
else if(is_operator(item) == 1) /* means symbol is operator */
{
x=pop();
while(is_operator(x) == 1 && precedence(x)>= precedence(item))
{
postfix_exp[j] = x; /* so pop all higher precendence operator and */
j++;
x = pop(); /* add them to postfix expresion */
}
push(x);
push(item);
}
else if(item == ')')
{
x = pop();
while(x != '(')
{
postfix_exp[j] = x;
j++;
x = pop();
}
}
else
{
printf("\nInvalid infix Expression.\n");
}
i++;
item = infix_exp[i];
}
if(top>0)
{
printf("\nInvalid infix Expression.\n");
}
if(top>0)
{
printf("\nInvalid infix Expression.\n");
}
}
int main()
{
int t;
scanf("%d",&t);
while(t--)
{
char infix[SIZE], postfix[SIZE]; /* declare infix string and postfix string */
scanf("\n%[^\n]s",infix);
InfixToPostfix(infix,postfix); /* call to convert */
//printf("Postfix Expression: ");
printf("%s\n",postfix);
/* print postfix expression */
}
return 0;
}
This is the code to convert infix to postfix expression I am using multiple test cases the problem is the second time(second test case the postfix_exp is printing more than it should)
Input
2
a+b*(c^d-e)^(f+gh)-i
a(b+c)/d
Output
abcd^e-fgh*+^*+i-
abc+d/fgh+^*+i-
Can you tell me what is wrong because of which "fgh+^*+i-" the extra expression from the previous expression is being concatenated at the end of the output?
The postfix_exp seems to be overwritten but I was expecting a new assigned postfix expression. Why is it that earlier data(data from previous test case) is saved?
Please ask any further query in the comments section
You did not closed postfix_exp with the terminating '\0'. After the first iteration postfix will look like something like this
XXXXXXXXXXX0
After the shorter 2nd input it will look something like
YYYYYYYXXXX0
Where X represent the characters written in the first iteration and Y the ones which in the second. So when you print it it will contain the last iteration results too.
Add this postfix_exp[j] = '\0'; before the if (top>0) line.
Note that this is still undefined behavior since postfix array is unitialized and there is no guarantee that it will be zeroed out.
you should clos postfix_exp with the terminating '\0' by adding
Add this postfix_exp[j] = '\0';
before the if (top>0) line.
What does it mean by item=infix_exp[i++]; in the following C code? Line no 21. It is for infix to postfix conversion. As far as I know, here i is array index. But why is it incrementing without any loop?
This is the Code
#include<stdio.h>
#include<conio.h>
#define SIZE 100
int top = -1;
char stack[SIZE];
void push(char item);
char pop();
int is_operator(char symbol);
int precedence(char symbol);
void main()
{
int i;
int j;
char infix_exp[SIZE], postfix_exp[SIZE];
char item;
char x;
printf("\nEnter Infix expression in parentheses: \n");
gets(infix_exp);
i=0;
j=0;
item=infix_exp[i++]; /* HERE */
while(item != '\0')
{
if(item == '(')
{
push(item);
}
else if((item >= 'A' && item <= 'Z') ||
(item >= 'a' && item <= 'z'))
{
postfix_exp[j++] = item;
}
else if(is_operator(item) == 1)
{
x=pop();
while(is_operator(x) == 1 && precedence(x)
>= precedence(item))
{
postfix_exp[j++] = x;
x = pop();
}
push(x);
push(item);
}
else if(item == ')')
{
x = pop();
while(x != '(')
{
postfix_exp[j++] = x;
x = pop();
}
}
else
{
printf("\nInvalid Arithmetic Expression.\n");
getch();
}
item = infix_exp[i++];
}
postfix_exp[j++] = '\0';
printf("\nArithmetic expression in Postfix notation: ");
puts(postfix_exp);
getch();
}
void push(char item)
{
if(top >= SIZE-1)
{
printf("\nStack Overflow. Push not possible.\n");
}
else
{
top = top+1;
stack[top] = item;
}
}
char pop()
{
char item = NULL;
if(top <= -1)
{
printf("\nStack Underflow. Pop not possible.\n");
}
else
{
item = stack[top];
stack[top] = NULL;
top = top-1;
}
return(item);
}
int is_operator(char symbol)
{
if(symbol == '^' || symbol == '*' || symbol == '/' ||
symbol == '+' || symbol == '-')
{
return 1;
}
else
{
return 0;
}
}
int precedence(char symbol)
{
if(symbol == '^')
{
return(3);
}
else if(symbol == '*' || symbol == '/')
{
return(2);
}
else if(symbol == '+' || symbol == '-')
{
return(1);
}
else
{
return(0);
}
}
item=infix_exp[i++]; means to fetch ith element of the array to item, then increment i by 1.
It seems the line is there because the author of the code prefered to write item=infix_exp[i++]; twice (the another one is in line 59) to using while((item=infix_exp[i++]) != '\0').
item=infix_exp[i++]; is equivalent to
item=infix_exp[i];
i++;
except that latter has one more sequence point.
It is just the same as
item=infix_exp[i];
i = i + 1;
The statement where you are getting confused is
item=infix_exp[i++]
Before this line of statement if the value of i is 0 then in this line the value of i is also 0 but the value of i in the next line is 1.
This statement is not in loop but the value of i is used in a loop. Each time the loop iterates the value of i is increased by one. Besides, i++ has no relation with loop. If you want to make the effect of increment in the very next line then you can do so.
I've searched the site to see if there are any similar questions to mine, but I can't seem to find one which offers a working solution. Below is the code to a fully functional infix to postfix converter for arithmetic syntax.
The issue is that if the user enters a string such as 23+89, the infix output is 2389+. This is of course correct, however I wish to add separators between the operands to distinguish between 23 and 89. How can this be implemented with the array I am using? Do I need to add a whitespace character between the operands in the array? How can this be achieved?
Any insight will be greatly appreciated.
#include <stdio.h>
#include <stdbool.h>
#include <ctype.h>
#include <string.h>
#define MAX 100 // maximum number of input characters
int precedence(char x);
struct stack
{
char my_stack[MAX];
int pointer;
};
int precedence(char x)
{
if(x == '(')
return(0);
if(x == '+' || x == '-')
return(1);
if(x == '*' || x == '/')
return(2);
return(3);
}
int main(void)
{
struct stack S;
S.pointer = 0;
char c;
char output[MAX]; // stores output
int out_count = 0; // stores no. of characters of output
char data; // stores data being popped off the stack
printf("Enter an expression to check:\n");
fflush(stdout);
while(('\n' != (c=getchar())))
{
if(isdigit(c)) // if an operand is found print to screen
{
output[out_count] = c;
out_count++;
}
else if(c == '(') // if ( is found push on stack
{
S.pointer = S.pointer +1;
S.my_stack[S.pointer] = c;
}
else if(c == ')') // if ) is found pop stack until ( is found
{
while(!(S.my_stack[S.pointer] == '('))
{
data = S.my_stack[S.pointer];
output[out_count] = data;
out_count++;
S.pointer = S.pointer -1;
}
S.pointer = S.pointer -1; // pop the extra (
}
else if(c == '*' || c == '/' || c == '+' || c == '-')
{
if((S.pointer) == 0)
{
S.pointer = S.pointer +1;
S.my_stack[S.pointer] = c;
}
else
{
if(precedence(S.my_stack[S.pointer]) > precedence(c))
{
// pop stack
int i;
for(i=(S.pointer); i >= 0; i--)
{
if(c == '(' || c == ')')
continue;
output[out_count] = c;
out_count++;
}
S.pointer = 0;
}
else
{
// push c on stack
S.pointer = S.pointer +1;
S.my_stack[S.pointer] = c;
}
}
}
}
int j = S.pointer;
while(j != 0)
{
if(S.my_stack[j] == '(')
continue;
output[out_count] = S.my_stack[j];
out_count++;
j--;
}
output[out_count+1] = '\0';
int k=0;
while(output[k] != '\0') // go through output array and print
{
printf("%c", output[k]);
k++;
}
return 0;
}
23+89, the infix output is 2389+. This is of course correct,
Well, no. 2389 is not the same as 23 and 89.
You could either add whitespace, or an explicit "push" operator. HP calculators often use an up arrow ↑ symbol to show that operation.
I'm solve an exercise in which one of the functions has to translate infix notation to postfix notation. Here follows my whole code
#include<stdio.h>
#define MAX 100
char stack[MAX];
int top;
void compact(char Descomp[], char Compac[]);
void init_stack();
int push(char Elem);
int desempilha(char *Elem);
int priority(char Operator);
int arity(char Exp[], int position);
int translate_pos(char exp[], char exp_pos[]);
int main()
{
char Exp[MAX]; /* stores the expression read from stdin */
char Exp_compact[MAX]; /* stores expression without spaces */
char Exp_pos[MAX]; /* stores the expression after the translation for postfix*/
int indicator; /* indicate if an error occurred, 0 for NO ERROR and -1 for ERROR*/
indicator = 0;
printf("\nType the expression: ");
gets(Exp);
compact(Exp, Exp_compact);
indicator = translate_pos(Exp_compact, Exp_pos);
puts(Exp_pos);
return indicator;
}
/* compact function delete spaces within the expression read from stdin */
void compact(char Descomp[], char Compac[])
{
int i;
int j;
i = 0;
j = 0;
while(Descomp[j] != '\0')
{
if(Descomp[j] != ' ')
{
Compac[i] = Descomp[j];
i++;
}
j++;
}
}
/* initiate the stack by setting top = -1 */
void init_stack()
{
top = -1;
}
/* puts the element Elem in the stack */
int push(char Elem)
{
if(top == MAX - 1) /* Stack is full */
return -1;
top++;
stack[top] = Elem;
return 0;
}
/* remove the element in stack[top] and puts it in &Elem*/
int pop(char *Elem)
{
if(top == -1) /* stack is empty */
return -1;
*Elem = stack[top];
top--;
return 0;
}
/* Return the priority of an operator */
int priority(char Operator)
{
switch(Operator)
{
case '+': return 1;
case '-': return 1;
case '*': return 2;
case '/': return 2;
case '^': return 3;
case '(': return 4;
case ')': return 5;
default : return 0;
}
}
/* returns the arity of CONSTANTS + - * / and ^, for ( an ) is merely symbolic */
int arity(char Exp[], int position)
{
if(priority(Exp[position]) == 1)
{
if( (position == 0) || ( (priority(Exp[position - 1]) >= 1) && (priority(Exp[position - 1]) <= 3) ))
return 1;
else
return 2;
}
else if( (priority(Exp[position]) > 1) && (priority(Exp[position]) <= 4))
return 2;
else
return priority(Exp[position]);
}
/* reads an infix expression and returns postfix expression */
int translate_pos(char exp[], char exp_pos[])
{
int i;
int j;
int ind;
char trash;
i = 0;
j = 0;
ind = 0;
trash = ' ';
init_stack();
while(exp[i]!= '\0')
{
if(arity(exp, i) == 0)
{
exp_pos[j] = exp[i];
j++;
}
if(arity(exp, i) == 1)
{
switch(exp[i])
{
case '-':
{
exp_pos[j] = exp_pos[i];
j++;
}
case '+': trash = exp_pos[i];
}
}
if(arity(exp, i) == 2)
{
while((top != -1) && (priority(stack[top]) <= priority(exp[i])))
{
ind = pop(&exp_pos[j]);
j++;
}
ind = push(exp[i]);
}
if(priority(exp[i]) == 4)
{
ind = push(exp[i]);
}
if(priority(exp[i]) == 5)
{
while( (top != -1) && (stack[top] != '('))
{
ind = pop(&exp_pos[j]);
j++;
}
if(stack[top] == '(')
ind = pop(&trash);
}
i++;
}
while(top != -1)
{
ind = pop(&exp_pos[j]);
j++;
}
return ind;
}
The algorithm I used to translate the expression is
while there is token to be read;
read the token;
if token is a constant
push it to Exp_Postfix;
if token is '('
push it to stack
if token is ')'
pop from the stack all symbols until '(' be find and remove '(' from the stack
if token is an operator and its arity is 2
pop all operators with less or equal priority than the token and store then in the Exp_Postfix;
push token to the stack;
if token is an operator and its arity is 1
if token is '-'
push it to Exp_postfix;
if token is '+'
pass to the next token;
pop all remaining symbols in the stack and push then, in order, to the Exp_Postfix;
I compiled the .c archive using
gcc -Wall archive.c -o archive
and executed it. I give the expression
5+(6*9^14)
It the returned expression was
5
I do not now if the error is in my code or in solution to the problem.
There are an awful lot of problems, here, for instance:
compact() and translate_pos() leave Exp_compact and Exp_pos, respectively, without a terminating \0, so you're getting garbage printed out.
your arity() function is returning 2 for an opening parenthesis.
in the first switch statement of translate_pos(), you're missing break statements.
Your priority comparison in translate_pos() when arity is 2 is back to front.
When you're comparing operator precedence, you should treat an opening parenthesis specially, since it should have the lowest precedence when on the top of the stack.
You should have a lot more else keywords in translate_pos().
You're using gets(), which was always bad, and now has actually been removed from C.
I haven't exhaustively tested it, but here's a correct version that seems to work for all the test inputs I tried:
#include <stdio.h>
#include <ctype.h>
#include <assert.h>
/* Function prototypes */
void compact(char Descomp[], char Compac[]);
void init_stack();
int push(char Elem);
int desempilha(char *Elem);
int priority(char Operator);
int arity(char Exp[], int position);
int translate_pos(char exp[], char exp_pos[]);
/* Stack variables */
#define MAX 100
char stack[MAX];
int top;
int main(void) {
char Exp[MAX];
char Exp_compact[MAX] = {0};
char Exp_pos[MAX] = {0};
int indicator = 0;
printf("\nType the expression: ");
fgets(Exp, MAX, stdin);
compact(Exp, Exp_compact);
indicator = translate_pos(Exp_compact, Exp_pos);
puts(Exp_pos);
return indicator;
}
/* compact function delete spaces within the expression read from stdin */
void compact(char Descomp[], char Compac[]) {
int i = 0;
int j = 0;
while ( Descomp[j] ) {
if ( !isspace(Descomp[j]) ) {
Compac[i++] = Descomp[j];
}
j++;
}
}
/* initiate the stack by setting top = -1 */
void init_stack() {
top = -1;
}
/* puts the element Elem in the stack */
int push(char Elem) {
if (top == MAX - 1) /* Stack is full */
return -1;
stack[++top] = Elem;
return 0;
}
/* remove the element in stack[top] and puts it in &Elem*/
int pop(char *Elem) {
if (top == -1) /* stack is empty */
return -1;
*Elem = stack[top--];
return 0;
}
/* Return the priority of an operator */
int priority(char Operator) {
switch (Operator) {
case '+':
return 1;
case '-':
return 1;
case '*':
return 2;
case '/':
return 2;
case '^':
return 3;
case '(':
return 4;
case ')':
return 5;
default:
return 0;
}
}
/* returns the arity of OPERATORS + - * / and ^,
* for ( an ) is merely symbolic */
int arity(char Exp[], int position) {
if ( priority(Exp[position]) == 1 ) {
if ( (position == 0) ||
((priority(Exp[position - 1]) >= 1) &&
(priority(Exp[position - 1]) <= 3)) ) {
return 1;
} else {
return 2;
}
} else if ( (priority(Exp[position]) > 1) &&
(priority(Exp[position]) <= 3) ) {
return 2;
} else {
return priority(Exp[position]);
}
}
/* reads an infix expression and returns postfix expression */
int translate_pos(char exp[], char exp_pos[]) {
int i = 0, j = 0, ind = 0;
char trash = ' ';
init_stack();
while ( exp[i] ) {
if ( arity(exp, i) == 0 ) {
exp_pos[j++] = exp[i];
} else if ( arity(exp, i) == 1 ) {
switch (exp[i]) {
case '-':
exp_pos[j++] = exp[i];
break;
case '+':
trash = exp_pos[i];
break;
default:
assert(0);
}
} else if (arity(exp, i) == 2) {
while ( (top != -1) &&
(priority(stack[top]) >= priority(exp[i])) &&
stack[top] != '(' ) {
ind = pop(&exp_pos[j++]);
}
ind = push(exp[i]);
} else if ( priority(exp[i]) == 4 ) {
ind = push(exp[i]);
} else if ( priority(exp[i]) == 5 ) {
while ( (top != -1) && (stack[top] != '(') ) {
ind = pop(&exp_pos[j++]);
}
if ( (top != - 1) && stack[top] == '(') {
ind = pop(&trash);
}
}
i++;
}
while (top != -1) {
ind = pop(&exp_pos[j++]);
}
return ind;
}
Gives the following output for various test cases:
paul#local:~/src/c/postfix$ ./postfix
Type the expression: 1+2
12+
paul#local:~/src/c/postfix$ ./postfix
Type the expression: (1+2)
12+
paul#local:~/src/c/postfix$ ./postfix
Type the expression: 1+2*3
123*+
paul#local:~/src/c/postfix$ ./postfix
Type the expression: (1+2)*3
12+3*
paul#local:~/src/c/postfix$ ./postfix
Type the expression: (3+4)*4/2
34+4*2/
paul#local:~/src/c/postfix$ ./postfix
Type the expression: 5+(6*9^14)
56914^*+
paul#local:~/src/c/postfix$
I'd suggest comparing my code to yours and trying to understand the individual differences to see where you were going wrong.
This is the code for reverse polish notation. When I run it on unix it works fine but codechef says wrong answer. Please help.
#include<stdio.h>
#include<string.h>
void push(char);
void pop();
char stack[400];
unsigned long top=0;
int main()
{
unsigned long len, test_case,i=0,j=0;
char expr[400];
scanf("%u\n",&test_case);
for(;j<test_case;j++)
{
scanf("%s\n",expr);
len=strlen(expr);
for(;i<len;i++)
{
if(expr[i]=='+'||expr[i]=='-'||expr[i]=='*'||expr[i]=='/'||expr[i]=='^')
push(expr[i]);
else if(expr[i]==')')
{
pop();
}
else if(expr[i]=='(')
continue;
else
printf("%c",expr[i]);
}
}
return 0;
}
void pop()
{
if(top==-1)
return;
else
{
printf("%c",stack[top]);
top=top-1;
}
}
void push(char x)
{
if(top==400)
return;
else
{
stack[++top]=x;
}
}
Change this line
for(;i<len;i++)
to
for(i=0;i<len;i++)
and then it will work fine. In codechef there are many testcases, so in your program it will work fine for 1st testcase but it will fail after that because the value of 'i' from the 2nd testcase will start from where it ends in the 1st case. So,you have to initialize the value of i in every testcase.
I went through the same situation and this is my solution to that problem:
#include<stdio.h>
#include<string.h>
char stack[400];
unsigned long top = 0;
void pop() {
if (top == -1) {
return;
} else {
printf("%c",stack[top]);
top = top - 1;
}
}
void push(char x) {
if (top == 400) {
return;
} else {
stack[++top] = x;
}
}
int main() {
unsigned long t, n, i = 0, j = 0;
char a[400];
scanf("%lu", &t);
while(t--) {
scanf("%s", &a);
n = strlen(a);
for (i = 0; i < n; i++) {
if (a[i] == '+' || a[i] == '-' || a[i] == '*' || a[i] == '/' || a[i] == '^') {
push(a[i]);
} else if (a[i] == ')') {
pop();
} else if (a[i] == '(') {
continue;
} else {
printf("%c", a[i]);
}
}
printf("\n");
}
return 0;
}
You missed writing some sections and there is another notation along with +, -, * and / i.e. ^ which was also required for the evaluation of the answer.
Only place that I noticed for the time being
`scanf("%u\n",&test_case);` //"%u" for unsigned not unsigned long
`scanf("%s\n",expr);` //%s : newline to not input are separated by white space
you have to initialize unsigned long i = 0 ; inside for loop as if you take two or more test cases i will initiate from where it left in last case for every next case .
Add
top=0;
stack[0]=0;
before ending for loop in function main and change
for(;i<len;i++) to for(i=0;i<len;i++).